A new hydrodynamic hybrid bearing and shaft seal has been launched into the hydropower sector by Wärtsilä. The bearing is manufactured from fibre and epoxy-based composite materials.
By David Appleyard
Finnish technology company Wärtsilä has a long history in the marine business. Supplying bearings and seals in extremely high-performance applications, such as the UK’s nuclear submarine fleet, it believes it has impressive credentials in the long-term reliability that is a basic requirement for the hydropower industry.
This is the view of Ross Strickland, consultant engineer at Wärtsilä’s UK seals and bearings division, who explained some of the company’s strategic objectives while outlining the key features of the company’s latest offering for the hydropower sector – a hybrid composite bearing, housing and seal system for hydropower turbine shafts. “We’ve got some pedigree in that area. Having supplied stern shaft bearings for 50-odd years, we’re on 23 navies and supply to the MOD [UK Ministry of Defence],” says Strickland.
And for the company, the move into hydropower bearings and seals makes sense, adds Les Creak, UK-based business development manager for Wärtsilä’s Hydro and Industrial segment, noting that Kaplan turbines, for example, look all but identical to a ships’ controllable pitch propeller.
Following the July 2007 completion of the acquisition of Railko Ltd Marine Products, which was founded in 1957 and specialized in synthetic or composite stern tube and rudder bearing technology, Wärtsilä has expanded its capabilities, adapted the technology and applied it to the hydropower sector.
|One of Wärtsilä’s vertical turning lathes used to machine composite bearings and housings.|
Composite technologies offer an alternative to white metal, bronze, wood and other materials that are currently applied in the hydropower sector and a number of other companies offer composite bearings.
Prior to the acquisition, net sales of the Railko’s marine business was about €6 million. Manufacturing today is located at a custom-built site on industrial estate in Slough, about 20 miles west of London.
It produces the Wärtsilä REsafe WCS bearing series, the company’s latest generation of water lubricated, composite bearings. Manufactured in a wound fibre or filament process, the technique offers attractive mechanical characteristics with relatively low mass.
Strickland explains: “The product consists of two elements. You’ve got the actual bearing itself, the material for which was developed for the marine market for stern shaft bearings, and the housing.”
“The [bearing] material is different to our traditional materials in that its epoxy based,” he continues.
It comprises a polymer fibre impregnated with resin that also contains a number of friction modifiers, including carbon, PTFE and other substances in an undisclosed formulation. A very low co-efficient of friction both wet and dry reduces the wear rate of the bearing, giving an extended operating life.
|A rack of test rig shafts of different metal grades used for in-house testing.|
Strickland says: “The friction you achieve between the shaft and the bearing is particularly low, the key thing is that it runs hydro-dynamically. When you get that hydro-dynamic operation, you get minimal wear to the bearing and the shaft journal. It comes down to the stability of the material and the friction. Because of the modifiers we’ve got in the resin, we have a very low friction on start up. You tend to get wear on start up when it’s not running hydro-dynamically.”
In addition, Wärtsilä says its composite bearings can achieve narrow running clearances due to low thermal expansion and water swell properties.
Turning to the housing, the wound filament process also offers an alternative to traditional materials such as spun-cast aluminium-bronze or steel. Structural composites can reduce the weight of the housing by up to 75% whilst maintaining structural integrity. However, in this case the bearing and housing may be manufactured as a single piece.
The housing is composed of fibre glass filaments, again impregnated with epoxy, which allows integrated manufacture. Ross says: “They are both epoxy-based, you can combine the two materials. We wind the bearing material on first and then we wind the housing material, which is a lot stiffer and stronger, to give it the support.”
As both elements are epoxy-based, they are effectively homogenous and there are no issues with transition between the bearing and the housing structure.
The fibres are wound onto a steel mandrel of an appropriate size and then it is cured in an oven before machining into finished dimensions. Strickland adds that a sophisticated winding set up allows external profiling of the product, saving materials and processing steps.
“We can then split it and then you bolt the two halves together, so you can assemble it around the shaft.”
Summarising, Strickland explains: “What’s unique about this is the speed that we can actually make it, the weight and the cost.”
“Aluminium bronze, which would be the comparable metal used in a water environment, is about four times heavier. If you imagine you’re inside a turbine and you’re trying to install it, if you’re able to hand carry it that is an advantage.”
Indeed, Strickland refers to the original inception of the design: “This came about through a customer asking us to replace their current grease-lubricated metal bearing.”
|A small diameter composite bearing mid-machining on a vertical lathe at Wärtsilä’s custom manufacturing facility in Slough, UK.|
“Because of the space available, we looked at combining housing materials – which are also epoxy based but with a glass fibre yarn – and incorporating the bearing with the housing. Traditionally they would be two separate components and you would have the bearing split and retained by typically bronze keys, enabling you to remove the bearing with the shaft in situ. With turbines that’s also quite common, because there’s a lot of work involved in removing the shaft, so we looked at this idea.”
And addressing potential concerns over the strength of composite materials, Strickland says: “It’s not as strong as bronze but better fits the loading characteristics the bearings will see in application; with a proof strength of 150Mpa it enables the bearing designs to be lighter, less cost, easier to manufacture yet fulfil all of the operational demands. In that sense composites are better suited to the future demands of cost and customisation than more traditional materials.
“When you’re looking at composites it allows you to use lighter materials, potentially cheaper and easier to manufacture, but still strong enough to do the job. So if you look at aerospace and similar industries, they push the boundaries of what they’re trying to do. They have smaller factors of safety, but are still more than capable of doing the job.”
While both housings and bearings can be fully split for easy assembly with the shaft left in situ, another feature is that these bearings can be provided as a packaged system incorporating a water-lubricated fully split seal.
Available as bushes, shells or segments they can also be supplied with sensors to track performance like wear and temperature.
|Cured billets of REsafe bearing material ready for machining.|
“We typically use a PT100 temperature sensor, they’re common across the marine industry. We’ve also got wear down devices, so we can measure proximity wear down of the shaft relative to the sensor. If it’s a turbine it’s slightly more complicated, because it can move in any direction, so you have to take multiple readings to give you an idea. We are exploring other avenues of condition-based monitoring too,” says Strickland.
The company also conducts extensive in house testing, with a fully equipped materials testing laboratory and a test rig. This set up uses a variety of different shaft materials in a horizontal configuration. With the shaft rotating, lubricating water flow rates can be adjusted or suspended solids can be introduced, for example.
Looking ahead, Creak says: “A driver is to advance our capabilities in split composite materials and material replacement.” Strickland concurs, saying: “I think the materials are where our strengths lie.”
David Appleyard is Chief Editor of HRW – Hydro Review Worldwide
Case study: La Tzintre, Switzerland
Turbines installed at the La Tzintre project near Charmey, in Switzerland, are one of the first hydropower applications to use Wärtsilä’s new composite bearing and housing system. Majority owned by Groupe E, the plant is run-of-river with a low gross head of less than 11 metres and a flow rate of around 10 cubic m/s.
It features twin horizontal axis machines with a combined capacity of 880 kW and produces some 3.4 GWh annually.
It forms part of Group E’s generation fleet, which includes eight hydroelectric plants.
The existing bearing required a fairly large amount of grease to be injected. In addition, depending upon the season, tree branches or other debris could get stuck between the runner blades and the runner housing. This saw bearing temperature rise abruptly – in a matter of seconds – shutting down the units. The operator suspected that these run-on episodes had damaged the bearing because the runner was no longer centred inside its housing. Despite these issues, vibration remained under 2.5 mm/s and normal bearing operating temperature was about 12 °C with little seasonal variation. There was also significant leakage through the shaft seal, which comprised of a standard stuffing box.
The bolt-on upgrade to water-lubricated composite bearings with an overwound structural composite housing and an integrated PSE-type mechanical seal was installed in November 2014. It removed the need to inject a large amount of grease, making the turbine more environmentally compliant, while changing the shaft seal from a packing box to a mechanical seal improved efficiency and reduced leakage.
According to Groupe E’s Rafael Valladares: “The behaviour of the shaft seal and guide bearing has been outstanding since the very beginning. No issues at all since the installation up to now [July 2015]. Temperature in the bearing is low and stable. No noticeable leakage on the shaft seal either. No more grease to deal with, no more contamination even if the grease was biodegradable.”